One Man's Quest To Find The 'Sonic Wonders Of The World'

Why does thunder rumble? Acoustic professor Trevor Cox explains that it has to do with the way lightning is a jagged line. "Each little kink is actually generating the sound, and the reason thunder rumbles is because the sound takes different time to come from different kinks because they're all slightly different distances from you," he says.

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Originally published on February 19, 2014 2:38 pm

Ever wonder why your voice sounds so much better when you sing in the shower? It has to do with an acoustic "blur" called reverberation. From classical to pop music, reverberation "makes music sound nicer," acoustic engineer Trevor Cox tells Fresh Air's Terry Gross. It helps blend the sound, "but you don't want too much," he warns.

Cox is the author of The Sound Book: The Science of the Sonic Wonders of the World. He has developed new ways of improving the sound in theaters and recording studios. He's also studied what he describes as the sonic wonders of the world — like whispering arches and singing sand dunes. His sonic travels have taken him many places, including the North Sea, where he recorded the sound of bottlenose dolphins underwater, and down into a revolting Victorian era sewer, where he discovered a curving sound effect he'd not heard before.

Interview Highlights

On how a gunshot in an anechoic chamber — a room that absorbs the reflection of the sound — is quieter than a snap

You walk into this chamber through some very large steel doors and the first thing that kind of strikes you is you're [standing] on a trampoline floor of wire, because all around you on the floor, on the walls, and on the ceiling are these strange gray foam wedges, and they're there to completely absorb any reflection. So when you hear the gunshot all you hear is the shot coming straight from the gun, or in this case, the microphone and there's no effect of a room at all. ... We don't normally hear sounds without the effect of a room. We normally have walls around us reflecting sound.

On whispering galleries

There's a variety of them around the world. In England the most famous one is St. Paul's Cathedral in London, where you go up to the base of the dome ... and you whisper into the wall and the sound skims around the inside of the dome to be heard by your friend who is way, way across on the other side. It sounds like the sound is emerging from the walls and you can have a conversation 100 feet apart from each other.

On how to design a concert hall

We want this reverberance — this sense [that] the sound lingers a bit after the orchestra finishes for a couple of seconds. [It] is typically a design criteria. And to do that, concert halls are really very large. So normally, if you sit in a concert hall and look up you'll see it's a huge volume, really high ceiling, that's to give you that reverberance.

But also we want to get rid of things which deaden the sound, you know, soft stuff like curtaining and carpets are all bad news. If you look around a concert hall, all the walls and the ceiling are really hard materials. There's a sort of myth around the concert halls made of wood that wood vibrates but if you were to take the wood off you'd find it's glued to concrete behind. There's no way that wood is vibrating at all.

And a third, key criteria is to make sure we get lots of sound from the sides. If you ever hear one of those concerts when they play outdoors, it sounds a bit remote, like the people are playing on the stage from a distance. In a concert hall, you're kind of surrounded by sound, that sense of envelopment is created by sound coming from the side.

On how sound travels underwater

[Sound travels] more efficiently [in water] than in air. It's the same kind of process in the fact that you've got a wave, and air is being passed from air molecule to air molecule, whereas in water it's being passed from water molecule to water molecule. It just happens to be that in water it goes further, which is another reason why aquatic animals like to use it — because it can travel huge, great distances underwater in a way that it never would do in the air. It would die away much quicker.

On why thunder "rumbles"

What's amazing about thunder is when you hear it, it's actually got that crack and then it's got the rumble afterwards. As a kid, when you drew thunderstorms you would've drawn the lightning with that jagged line. If you didn't have that jagged line, you wouldn't have the rumble of the thunder.

... The visual look of lightning is really crucial to how the thunder sounds. ... Each little kink is actually generating the sound, and ... the sound takes different time to come from different kinks because they're all slightly different distances from you. That's the reason you get that very distinct rumble sound.

Copyright 2014 NPR. To see more, visit http://www.npr.org/.

Transcript

TERRY GROSS, HOST:

This is FRESH AIR. I'm Terry Gross. This interview is going to start with a bang, in fact a gunshot. But don't worry, the gun is just to demonstrate an acoustic phenomenon. My guest, Trevor Cox, is an acoustic engineer. He's developed new ways of improving the sound in theaters and recording studios. He's also studied what he describes as the sonic wonders of the world, like whispering arches and singing sand dunes.

His sonic travels have taken him many places, including the North Sea, where he recorded the sound of bottlenose dolphins underwater and down into a revolting Victorian era sewer, where he discovered a curving sound effect he'd not heard before.

Cox is a professor of acoustic engineering at the University of Salford in Manchester, England, and the author of the book "The Sound Book: The Science of the Sonic Wonders of the World." He's brought a few recording of sounds few people have ever heard. Trevor Cox, welcome to FRESH AIR.

TREVOR COX: Thanks.

GROSS: So let's start with a sound of a gunshot. Now, this is a gunshot that you recorded in what's called an anechoic chamber, a chamber with no echo. I want you to describe what this chamber is.

COX: Well, you walk into this chamber through some really large steel doors, and the first thing that kind of strikes you is you stood on a trampoline floor of wire because all around you on the floor, on the walls and the ceiling, there are these strange, gray foam wedges. And they're there to completely absorb any reflection.

So when you hear the gunshot, all you hear is the shot coming straight from the gun to you, or in this case the microphone, and there's no effect of a room at all.

GROSS: So I'm going to play what a gun sounds like fired in a room with absolutely no echo. And I must say it's going to be a pretty unimpressive sound. Here it is.

(SOUNDBITE OF GUNSHOT)

GROSS: Well, that really was mighty unimpressive. My finger snap...

(SOUNDBITE OF FINGER SNAP)

GROSS: ...is more impressive than that in our studio, which is relatively - you know, it's meant to be a soundproof studio. Why does it sound - you know what? Let's play it one more time.

(SOUNDBITE OF GUNSHOT)

COX: You see, I think you assume that's unimpressive but actually I'm really impressed by it because you think of how you normally hear a gun, especially when Hollywood has had their hands on it, how impressive everything sounds and how it booms in the room. And as you said, when you click your finger, you get this booming sound. A lot of that is what the room is doing.

We don't normally hear sounds without the effect of a room. We normally have walls around us reflecting sound. So it's kind of - it takes you back. I was quite - you know, when I first recorded that, I thought, did I get the recording right, did I fire the gun...

(LAUGHTER)

COX: Because it's just like a click.

GROSS: OK, so let's do the opposite. You also fired a gun in a room designed to have a lot of reverb. Describe that room for us.

COX: Well, this is quite an incredible place. It's actually an old oil tank. It was buried in the side of the Scottish hills back at the start of the second world war to protect all the shipping oil from the German bombing. And so it's a vast space. It's kind of a size of a cathedral. And it's just built out of this vast concrete arch.

And yeah, we fired a starting pistol in there, exactly the same one you just hear in the anechoic chamber, to measure the acoustic.

GROSS: OK, let's hear that one.

(SOUNDBITE OF GUNSHOT)

GROSS: Wow, it's still echoing. Gee. You know, you know what? It sounds like this really loud gun turning into a plane going by.

COX: Yeah, it's quite incredible when you're in there as well, because you actually, you can feel the sound wash over you.

GROSS: It's still echoing. I still hear it in the background. It's still going.

COX: It is still going. When I was doing the recording, of course I was staying completely silent to get all the recording, and I just was watching the digital meter counting up time, and the recording you've got there, it's over a minute long before it died away. And funny enough, it was still dying away when I turned it off. But it was below the threshold of hearing. It was below what I could hear anymore at low frequency, but it was still going when I stopped.

GROSS: So let me just ask you if, if you're in this like huge metal tank, basically, how do you prevent the gun from ricocheting - the bullet from ricocheting?

COX: Oh, well, that was a starting pistol. So it was firing blanks. So it was...

GROSS: Oh, of course, right.

COX: It's quite a standard method of measuring concert halls, actually. We would do it in a normal sort of classical concert hall to measure the acoustics. And it's a really quick and easy way. I mean this tank, which is, as I said, in the middle of nowhere in Scotland, right at the top of Scotland, is also got no power in it. It's no lighting. It's got no electricity.

So you have to be able to measure it with anything - something really easy. So there's no power for a loudspeaker, for example. So we used a gun to excite the space as a way of measuring. But we also had a limitation as well, because the only way into is down the oil pipes themselves. There are no doors into this place.

So you placed in a horizontal pipe and shoved through a pipe, which isn't much bigger than you are, to actually get into this oil tank.

(LAUGHTER)

GROSS: You know, I don't really think of acoustic engineers as doing this kind of thing. But you're just obsessed with sound.

COX: Yeah, I mean actually my whole adventure kind of started in a sewer of all places. And I would not recommend going into a sewer. But I was asked to do an interview on the acoustic of a sewer, and I thought, well, I've never been in one. I'll go and see what it's like. And it was as revolting as you can imagine.

But it had this most amazing acoustic effect. It was a cylindrical sewer, an old Victorian brick storm drain, and actually the sound would cling to the inside of the brickwork and spiral around and around in circles, and it was quite amazing.

GROSS: So what did you learn acoustically from that smelly and disgusting but audio fascinating trip?

COX: Well, I think the first thing I took from that was actually some amazing acoustics can be in the most unlikely of places. And some of the other places I went to, like the oil tank, are not immediately on your list of places to go to if you're thinking of acoustics, or nice places for that matter.

But the other thing is that spiraling effect, what I realized was that's the kind of same effect as you get in whispering galleries. There's a variety of them around the world. In England the most famous one's St. Paul's Cathedral in London, where you go up into the base of the dome of St. Paul's Cathedral, and you whisper into the wall, and the sound skims around the inside of the dome to be heard by your friend, who's way, way across on the other side.

And it sounds like the sound is emerging from the walls. And you have a conversation a hundred feet apart from each other.

GROSS: So let's get back to those two different spaces that we heard the gun, the starting pistol, fired in. The first was a room specially designed to have no echo at all, and the second was this huge oil drum that is incredibly reverberating, lots and lots of echo. And we heard the difference of how a starting gun sounds fired in each.

What's the moral of that story in terms of the kind of acoustics that you actually make your living doing, which is designing acoustics for theaters and concert halls?

COX: Yeah, I mean those two examples are just interesting extremes. I mean, that oil tank is now the world record holder for its reverberation. And the anechoic chamber is the complete opposite. But actually when you design a room, you would never go to those two extremes because it's just not necessary, and it's also not very good.

So if I wanted to design a place for music, let's say classical music, then I want a bit of reverberation because it makes music sound nicer. I mean it's a standard kind of thing that people even making pop music do is when they hear vocals, they put reverb on it just to make the sound blend a bit better.

You know, when we go into a shower, we sing because we sound a little better with a little bit of that sort of blur going on. But you don't want too much. So that oil tank is too much because if you really reverb, then what happens is you get a mush, all the words and all the music just run into each other. So normally you would never have anything as reverberant as that oil tank or as dry as the anechoic chamber.

GROSS: So when you're designing a concert hall that's going to hold symphony orchestras and also rock concerts, how do you design something that's going to be suitable for both?

COX: Well, there's a few key things that we know are really important for a - let's say classic music to start with. We know that we want this reverberance, this sort of kind of sense that sound lingers a bit after the orchestra finishes, for a couple seconds is typically a design criteria. And to do that, concert halls are really very large, so normally if you sit in a concert hall and look up, you'll see it's a huge volume. It's really high ceiling. That's to give you that reverberance.

But also we want to get rid of things which deaden the sound. You know, soft stuff like curtaining and carpets are all bad news. So actually if you look around a concert hall, all the walls and the ceiling are really hard materials. There's a sort of kind of myth around that the concert halls are made of wood so that the wood vibrates. But actually if you were to take the wood off, you'd find actually it's glued to concrete behind. There's no way that wood is vibrating at all.

And a third sort of kind of key criteria would be also to make sure we get lots of sound from the sides because when we're outdoors - have you ever heard one of those concerts when they play outdoors? It all sounds like a bit remote, like the people are playing on the stage from a distance. In a concert hall you're kind of surrounded by sound. That sort of sense of envelopment is created by sound coming from the side, which is why halls, you know, something like Boston Symphony Hall is narrow and tall.

It wasn't - we didn't know that when it was designed back in its day, but we now know why it works, is because it creates lots of sound from the side.

GROSS: If you're just joining us, my guest is Trevor Cox. He's an acoustic engineer and the author of the new book "The Sound Book: The Science of the Sonic Wonders of the World." Let's take a short break; then we'll talk some more. This is FRESH AIR.

(SOUNDBITE OF MUSIC)

GROSS: If you're just joining us, my guest is acoustic engineer Trevor Cox. He is a professor of acoustic engineering at the University of Salford in England. He's the author of the new book "The Sound Book: The Science of the Sonic Wonders of the World."

So we've been talking about the acoustics in concert halls and theaters. You say that cathedrals have natural reverb in it. Do you think - like what makes cathedrals so great for sound, old cathedrals?

COX: Well, there's a couple of things. I think, first of all, from the point of view of, you know, a place to have a religious ceremony, I think we associate it with something very spiritual. And that's just to do with the very reverberant, echoey conditions in there, because if you go in there, and you start talking too loud, it echoes around and you're suddenly embarrassed.

So it's kind of that very sort of hard surface, big volume, kind of forces us to whisper and therefore treat the place with a bit of reverence. So I think the spiritual sense in a church is really kind of dictated by the sort of conditions of these old Gothic cathedrals.

And we kind of seem to associate that with a sense of spirituality.

GROSS: Let's go back to the anechoic chamber, and that's the soundproof chamber with, like, no echo at all that we started talking about at the beginning of the interview. And this is the one where you fired the starting pistol, and it just sounded like a little pop.

So does your ear, when you're in a place that's that soundproof, does your ear or your brain try to compensate for the lack of sound and find something to hear?

COX: Yeah, it's one of the surprises when you take visitors into that anechoic chamber, is that they actually hear sound even though you say we're taking you into a completely silent room. And actually the sounds that you're hearing are ones you generate yourself.

So you know, you've normally got your blood pumping through your body, including in your head, and normally that's not audible. But, you know, sometimes when you exercise, you get that pounding effect. So you can hear a bit of blood pumping sometimes in the anechoic chamber.

And you can also sometimes hear a high-pitched hissing sound, and that's thought to be sort of the firings in the auditory nerve. I mean essentially what happens is your brain is tweaking the inner ear to try and pick up sound. It's kind of turning the volume up, really, of the inner ear. And it turns it up so far that you start hearing a bit of the nervous system working.

So, you know, you can - there's a couple of sounds you hear, and that was what inspired John Cage's famous silent work "4 Minute 33 Seconds." He went to an anechoic chamber across in the States at Harvard expecting find silence and then didn't find it, and that inspired his composition.

GROSS: And that composition is four minutes and 33 seconds of just silence. Like he begins the piece, nothing happens. You're listening to the ambient sound around you. And then it ends after four minutes and 30 seconds. And it's kind of like an awareness composition of just, like, being aware of the sounds around you.

COX: Yeah, it's a very interesting thing to go and hear. Actually, I went and heard a performance of it. And I mean the thought of paying to hear nothing sounds a bit odd in the first instance, but actually it was quite - it was a fascinating thing. And what happens is just like a music performance. So we had a pianist playing it, and they come on and then they bow and you clap, and all the standard stuff goes on except for nothing is actually played.

And what becomes apparent as you listen to it is it's really all about the noise of audience. So it's - the performance is no longer really focused on the stage; it's focused on the people around you. And at the end of the performance, there was a round of applause and a sort of few ironic encores being shouted from the audience.

And funny enough, I felt a real sense of collective endeavor. And that's what you get when you listen or make music, you get a sense of collective endeavor. So even though it's a slightly strange piece, it certainly felt like a piece of music.

GROSS: When you spoke, I'm sure you tried speaking in the anechoic chamber, the soundproof room, did your voice sound different than you're used to?

COX: Yeah, it sounds very different, and in fact some visitors find it quite unpleasant. It's a bit like when you go up in an aircraft and your ears need to pop. It sounds a bit like that. So all the voices are a bit muffled. And if you have to talk, you find yourself straining a bit. You find yourself working a little harder as you talk.

And the thing that visitors don't like about it is because you see a room. It's very clearly - there's the walls all around you. They look odd, but they're clearly there. But your ear can't ear the room. And so your senses are a bit off-kilter. Your sense of vision and sense of sound are out of kilter. And some people feel a bit queasy and ask to leave.

GROSS: What's the purpose of the room?

COX: Well, if you want to measure anything in acoustic engineering, then you need to be able to sort of kind of isolate the effect of objects. If I had a loudspeaker, if I stick it in a room, I measure the loudspeaker, but I also measure all the effects of the room. If I take it into an anechoic chamber, I measure just the effects of the loudspeaker.

So it's a way of isolating things, and it has to be incredibly quiet because we also measure very quiet things in there. So it's - it's really it's like an isolation booth for acoustic measurement.

GROSS: You know, when you're - I'm sure coming out of a room with no sound, like a totally soundproof room like the anechoic chamber, when you emerge from that, you probably realize that there's just about no such thing as silence in the actual world, that there's always ambient sound around you, even when you think there's silence.

COX: Yes, I think finding silence is actually quite difficult in the modern world. And the funny thing about the anechoic chamber for me is if you go into it a lot, because you've been there a lot of times, your brain gets used to it. So it knows it's an odd room. For me, it's just like walking into any other room now. So that startling sort of effect of coming out and suddenly your ears opening up to the sounds and suddenly things are less muffled, I don't get that anymore.

It's only something you can really appreciate the first time in the space.

GROSS: So one of the things that you discuss in your book is the sonic wonders that are happening out of our hearing, like the sounds underwater. And as an example, you actually brought an example with you of a recording made underwater. And I'd like you to introduce this for us and tell us what we're going to be hearing.

COX: Yeah, this is recorded up near the Arctic, under the Arctic ice. And it's actually bearded seals calling out to each other. And to make this measurement, you need a hydrophone. I'm very lucky that Cornell University gave me a copy of this because this is one I haven't got to hear myself yet.

GROSS: You mention a hydrophone. This is like a microphone under water?

COX: Yeah, it's a microphone which works underwater. I mean, our hearing doesn't work very well under water. If you go swimming, you know that. And similarly, normal microphones don't work very well underwater, and you need special microphones designed to work in water.

I mean aquatic animals use sound all the time because light doesn't travel very well in water. It soon disperses. It's hard to see what's going on. But sound can travel huge distances. And that's one of the things these bearded seals are doing, is they can communicate over huge distances with these low-frequency, sort of weird glissandos.

GROSS: OK, so let's listen, and then you can tell us more about it.

(SOUNDBITE OF SEALS)

GROSS: I have to say that's the kind of sound you'd expect to hear in a science fiction movie.

COX: Yeah, I mean it sounds like UFOs landing or something, doesn't it? I mean, there are some animals which make the most strange sounds.

GROSS: And those were seals?

COX: Yeah, they're bearded seals. And in - when you're - the males calling out to the females and quite possibly saying, you know, come over here, I'm the best male to mate with. And the thing is, the male's attracting the female by doing a song. And so what it does is it tries to show its vocal athleticism to show it's really particularly fit to be mated with.

And I guess one of the things they might do is over time extend their glissandos longer and longer and longer because it's more and more impressive to the female. And it's common in animals. You know, you have these most strange sort of calls. But a sort of, kind of - it's a way of attracting the female or defending their territory.

GROSS: So it's just amazing to think about all the sounds in the world that the human ear doesn't hear because either it's out of, you know, we're not underwater listening, or because it's out of the range of our hearing. But let me ask you. I don't know if you know the answer to this, but if you were - if you somehow managed to dive underwater in Alaska, where these seals are, where it's probably way too cold to dive, would you be hearing those glissandos?

COX: Well, your ear is not particularly sensitive because it's not designed to sort of have waterways lapping against our eardrum and working. It's designed to have air pushing against our eardrum. So we certainly can hear something when we're underwater because of course if you go, I don't know, go into the swimming pool, you'll hear the radio on a side being sort of kind of still picked up. But it is quite muffled.

So I would think that you probably could hear something, but it wouldn't be as impressive as if you picked it up on a hydrophone.

GROSS: Right, so a hydrophone is designed to hear the things that the human ear wouldn't hear underwater.

COX: Yeah, I mean a very common tool, if you're, say, the military wanting to sort of listen out for what's around, or if you're a fisherman wanting to send sonar signals out to try and find fish, or if you're, you know, oil exploration, and you wanted to kind of do seismology kind of testing, you need things in the water to pick up sound, and that's what the hydrophone is for.

GROSS: Trevor Cox will be back in the second half of the show. His new book is called "The Sound Book: The Science of the Sonic Wonders of the World." I'm Terry Gross and this is FRESH AIR.

(SOUNDBITE OF MUSIC)

GROSS: This is FRESH AIR. I'm Terry Gross back with Trevor Cox, author of "The Sound Book: The Science of the Sonic Wonders of the World." He's an acoustic engineer who's designed sound for theaters and concert halls. He's also traveled extensively, documenting sonic wonders. When we left off, we were talking about a recording of bearded seals near Point Barrow, Alaska calling out to each other under water. The seals were recorded with a special microphone designed for underwater use called a hydrophone.

Well, another recording you brought with you is of tadpoles munching on a hydrophone. Why would they be doing that?

COX: Yeah. This was - I met an artist called Lee Patterson who's local to me in Manchester, and he likes to go and record in little sort of fishing lakes, not very big places, and pick up the sounds of tiny animals. So things like, you know, like water boatman or pond skaters, you know, they make - sort of sound a bit like crickets. But he - when he - when I met up with him, there were loads of tadpoles. It was in spring. And if you drop a hydrophone into them, the tadpoles will come along and try and eat it.

And so what you hear is the sound of the sort of scraping of the tadpoles - they don't really have teeth but their sort of mouth parts - on the hydrophone. And it's a bit disconcerting because he handed me a pair of headphones to put on to listen to it because we were by the lake, and it really did sound like someone was scraping right at my ear. So it was a bit disconcerting.

GROSS: Well, let's hear how disconcerting it is. This is tadpoles munching on a hydrophone.

(SOUNDBITE OF TADPOLES MUNCHING ON A HYDROPHONE)

GROSS: That is so odd.

(LAUGHTER)

GROSS: Right, it is. And when you consider how tiny - I don't know how amplified that is, but when you consider how tiny tadpoles are, that they'd be making that kind of munching sound?

COX: Yeah. You have - it is - it's highly amplified, actually. And yeah, he's - because he's perfected his techniques for picking up these tiny little sounds. So yeah, you have to work quite hard to get that. But it is quite an amazing sound. And the world is full of these natural sounds. We think of natural sounds. We think of things like birds songs, but there's a whole variety of kind of crazy sort of animal sounds that you can pick up if you look out for them.

GROSS: How does sound travel underwater?

COX: Well, much more efficiently than it does in air. I mean, it's the same kind of process in the fact that you've got a wave, and in air, it's being passed from air molecule to air molecule whereas in underwater, it's being passed from water molecule to water molecule. And it just happens to be that in water it goes further, which is another reason why aquatic animals like to use it because it can travel huge, great distances underwater in a way that it never would do in the air. It would die away much quicker.

GROSS: Let's talk about one of the sonic wonders of the world, which is a really familiar sound, and that's just like thunder. As an acoustic engineer, tell us your version of what's happening with loud thunder.

COX: What's amazing with thunder is when you hear it, I mean, it's actually got that crack, and then it's got the rumble afterwards. As a kid, when you drew thunderstorms, you would've drawn the lightning with that jagged line. If you didn't have that jagged line, you wouldn't have the rumble of the thunder.

So it's quite interesting how, you know, just something - a visual look of lightning is really crucial to how the thunder sounds. So when you draw a jagged line like that, each little kink is actually generating the sound. And the reason that a thunder rumbles is because the sound takes different time to come from different kinks because they're all slightly different distances from you. And that's the reason you get that very distinctive rumble sound.

GROSS: Oh. I always figured the rumbling was because it was like echoing all around, you know, the buildings or the woods or wherever you were.

COX: Well, I mean, in woods you will get echoing. But no, this is - the rumble is due to the fact that the sound is made all away along the lightning strike, all away along the sort of bright light because it's - I mean, it's basically created when the air is suddenly heated up. You get all these shockwaves generating. But because that strike, you know, those long, you know, bright light is all different distances from you, some of it arrives quickly from you because it's near you, and some of it arrives slower because it's further away.

So you get this sort of extended sound. If you've ever been near when the lightning strike, you don't hear the rumble. You just hear their loud crack, really, and that rumble really - only really is more obvious the further away you are.

GROSS: Why is wind noisy?

COX: Well, wind on its own isn't particularly - it's going to hit something to make it noisy. And at Manchester at the moment, the best example of that is the Beetham Tower in the city center. So we have this tower block in the city center which howls in the wind. And we've just - as well as having a really damp January, a mild January, we've had a really, really windy winter. And only a couple of days ago, it was so loud. It was waking people up in the town center.

And the wind itself was hitting - there's a structure on top of this Beetham Tower, which is a whole set of glass panes. And it was hitting those, and it was creating a whistling sound which is being amplified. So it's like some giant wind harp up here. And it creates all sorts of noise problems.

GROSS: So when you say that wind doesn't make - wind isn't inherently noisy, what creates the noise?

COX: What creates noise in the first place is turbulence. So if I was to pick up a beer bottle and blow across it, what you have is you got air sort of hitting the edge and creating sort of eddies. I mean, you see turbulence in the water. So if I have a smooth-running river and I put a rock in, you suddenly see that white water. That's turbulence of the water. The same thing is happening around the bottle neck. When I blow across it, you just can't see it obviously because it's sound and you can't see the air molecules moving.

And then that sort of turbulence creates a sort of - kind of a vague noise. But it will lock into the resonance of the air in the main base of the bottle. And that's what actually creates a distinctive tone. And it's how flutes work. It's how - if you ever hear wind whistling past railings, that's what you get. You hear an Aeolian harp, it's created by turbulence in a slightly different way, but it's - mostly, it's created by turbulence of some form.

GROSS: One of the things you've done - you have a very interesting website, and one of the things you've done is do a survey of what people find the most unpleasant noise in the world - the most unpleasant sound in the world. So what one?

COX: Well, in my survey, yeah, people went online and listened to a whole variety, about 30 other sounds, and they scored them. The worst one was the sound of someone being sick, I'm afraid, which...

GROSS: Somebody vomiting. Somebody retching.

COX: Yeah. I'm afraid yeah. People not often like me mention it really, but that was came at the top.

GROSS: So just curious, who did you record retching so that you could play it for people on this - for this survey on the Web?

Well, I must admit it was an actress. There were - no real sick was generated in making it. It was actually just actors doing it. But it was a very good recording. It was a very good simulation. And the reason we find this unpleasant is a disgust reaction. So we have this sort of very strong aversive reaction to things that might carry something like disease. And obviously, if someone is being ill, then potentially they've got disease. And we're sort of kind of - we immediately think, oh, I must keep away from that.

COX: And that's the reason it came top of the list, not only because it was just a superb recording or such a superb rendition. It was because we all universally have this sort of disgust reaction to avoid disease. Whereas you take fingernails down the blackboard, that scraping sound, which is the archetype worst sound, what we found is while some people found it really horrible, there are people who don't find it particularly unpleasant. So on average, it doesn't come top of the list.

GROSS: Did you do the flip and ask what the most pleasing sound was?

COX: I've always meant to do this experiment, but I've never actually done it. I mean, I think we know some of the stories. And in fact, you know, things like the warble of a bird song, you know, some really pleasant birds singing would be - probably come pretty high on lots of people's lists. But there's lots of them.

I mean, my favorite would probably be thinking about to when my children were younger and they used to chatter to each other and play along with each other. And you'd just listen to their voices chatting and doing their sort of imaginary play. It was always very delightful to listen to.

GROSS: If you're just joining us, my guest is Trevor Cox. He is an acoustic engineer and author of the new book "The Sound Book: The Science of the Sonic Wonders of the World." Let's take a short break, then we'll talk some more. This is FRESH AIR.

(SOUNDBITE OF MUSIC)

GROSS: If you're just joining us, my guest is acoustic engineer Trevor Cox, author of the new book "The Sound Book: The Science of the Sonic Wonders of the World." You know, earlier, we were talking about things that we don't know are making sounds underwater, and you played us the sound of seals underwater. It's also happening in the skies, things like bats with their sonar - these, you know, that they sent out to get a sense of where they are. Maybe you can explain why bats - what's happening with the bats, but I just want to say it's almost just quieting to think about all the sounds that are out there that are out of our range of hearing.

We like to think of ourselves as being perceptive and really knowing what's happening in the real world, but there's all the stuff happening in the real world that we're just not capable of perceiving.

COX: Yeah. It's quite interesting when you go and listen to bats. I mean, you have to take something along to listen to them. So you can buy bat detectors, which basically take the frequencies which are too high for us to hear and bring them down so you can hear them. And it's quite amazing to walk around at dusk and suddenly realize there's all these animals around that you never realized.

And the thing that natural history would call - as Chris Watson made to me - that these - you think, oh, that's interesting. It's a nice click. But actually, these bats are out and hunting. So it's actually like a battle zone out there in the ultrasonic region, you know? There's frequencies we can't hear. They're actually battling to try and actually find the insects and kill them. So it's not quite as such as kind of a nice and sort of nature sort of sound as you might think.

GROSS: So what is the sound that the bats emit and what is it for?

COX: Well, it's a very short chirp, mostly. So it's a sound which is quite short and sweeps causing frequency very quickly. And they're listening for the reflection coming back. I mean, they're actually basically shouting, almost - you know, they work at real limits of how their vocal system and how their hearing system works, which is basically the same as ours, pretty much.

So they're shouting these very short, little sort of chirps out. They bounce off objects, let's say, another insect or off a tree, then come back and by (unintelligible) what reflection is like, it can tell what it's reflected off, and also by how long it's taken, they can tell how far where the objects are. And that's how they navigate. I mean, there are people who do echolocate. As human beings, we do it. You can train yourself to do it if you wish. And there are blind people who do it. But it's a skill you have to learn. You can't just continuously pick up.

GROSS: Echolocate so that you figure out where you are based on the reverberations of the sound of your clap or a voice or whatever?

COX: Yeah. Most of the people who echolocate make a little click. You know, like, you would be going (makes noise) with your tongue or something like that. And then they hear the sound reflecting off the surfaces. And they can tell, you know, if they're close to a surface. That little click comes back slightly elongated so they know how far where the surface is. They listen to very subtle changes in the sound when they're looking at it. And, yes, it's amazing when you - when people do it because it's kind of a skillful thing to do and something you don't expect from a human.

GROSS: What's one of the sonic wonders of the world that you find most fascinating that I have not yet asked you about?

COX: It's really hard to pick a favorite - most amazing one, because there's just so many. But I must tell you, I really, you know, really enjoyed going to the Mojave Desert to hear the booming sand dunes. And I think when I first thought this idea of going to find this sort of amazing sounds in the world, half of my list was singing sands, because they had been written about for centuries.

You know, Marco Polo, there's ancient Chinese writing on them, Charles Darwin wrote about them. So they're been known about for some time. But also there's a lot of scientific controversy. There's been groups arguing about what causes them, which has also interested - tweaked my scientific mind. But to go and see one is not very, you know, you can't hear this. It's not very easy because you need to find a desert.

And there's no - I mean, the nearest one to England would probably be Morocco. But I hunt to be in America, so I went to southwest. There was quite a few around in the southwest. And the Kelso Dune to the Mojave Desert do it. So you climb up to this dune, which is really hard work. It's got to be really dry. It's got to be the heat of the summer. And the sand is really loose, because if it's not loose, it doesn't work.

And you see, it's made when you walk across it, you got this burping sounds, you know, a bit like a badly played tuba going, even just walking across it. But the sound that people write about is the sound what you create with an avalanche. You find the right slope and you scoot down on your backside, and you'll get this rumbling sound which drone that picks up a bit like a taxiing aircraft. And actually you can feel the whole surface vibrate underneath you. It's quite an amazing effect.

GROSS: So did you do that?

COX: Yeah. We were - we were unlucky on day one. We got - we arrived then - we couldn't it to work. And someone had said Kelso Dunes wasn't as tuneful as it used to be. So we went back and sort of evaluated, you know, overnight, looked back at, you know, some of the papers people have written about. And so it's kind of trying to work out. And I think the mistake we made on day one is we looked at this big dune and thought, all right, let's go to the top. We'll start there.

Well, actually, the best slope was about three quarters the way up. So on the second day, we found the one which was better angle to the wind. And the thing is what happens when these sand grains have to be all roughly the same size. They have to be, roughly, spherical. They have to have special varnish on them. So you see, there's a weird sifting happening of the - of the sand grains in the wind in the Mojave Desert.

And so you have to find one which is quite, you know, a bit of the dune, which is at the right angle to the prevailing wind direction as well. And I think, the first day, we weren't quite at the right angle. Day two, we got it better. We found a bigger slope. And we got some beautiful drones.

GROSS: So you actually have a recording of this singing sand dune. Do you want to introduce it for us?

COX: Yeah. There's two recordings actually worth playing. The first one is this burping effect, as it's described. And it's just me walking up the dune. You might even hear me panting a bit because it's quite hard work.

GROSS: OK. Let's go with that.

(SOUNDBITE OF SINGING SAND DUNES)

COX: The second sound which is the - this more famous droning sound is that me coming down on my backside and creating an avalanche. And it's quite low frequency, so you need to turn the bass up to hear it.

GROSS: And is that really an avalanche or just a sound of an avalanche?

COX: It's not just synchronized avalanche of all the drains. So all the drains move in one, and therefore create sort of a sound, which is very tonal. Normally, with an avalanche, it's just like a snow avalanche. Everything is just a kind of general sort of noise. But the - because the grains are all the same size, they slip past each other in synchronization. You get this amazing droning sound.

GROSS: OK. Let's hear it.

(SOUNDBITE OF SINGING SAND DUNE)

(SOUNDBITE OF HUM)

GROSS: Do you find it really weird when something makes a sound that it doesn't seem to be capable of making? Like why would sand make a sound like that?

COX: Yeah. To me when I was looking for these sort of sonic wonders, one of my things was, you know, what to go and see. I could go and see lots of things. And I think the sounds which are remarkable are some of the ones which are surprising. And to me, I've been to lots of sand dunes in my time but none of them make a sound. You know, I had to work quite hard to find this place.

And that's what makes it remarkable. There's only - there are about 40 documented around the world which make that particular sound. There's probably a few more to be found, but people don't tend to go into the middle of deserts to find them. So there's probably a few more but they're relatively rare. And also, it's just - you know, also it had a nice kind of sad story behind it, which was another reason why I wanted to visit.

GROSS: Since you're listening so carefully for things how do you take care of your ears?

COX: Very carefully when I can. So I play the saxophone and I have ear plugs with me, special musicians ones which I'll wear if I'm in a band and it's too loud. So, yeah. No, I do quite a lot of care of my hearing. Unfortunately, I have just developed tinnitus to do with a problem, not to do with noise exposure, which is very annoying.

GROSS: You've developed tinnitus?

COX: Yeah. I developed a ringing in the ear due to an infection in my ear which is a very annoying thing to, kind of, get as an acoustic engineer but unfortunately it's just one of those things. There's no real cure. So you just kind of put up with it.

GROSS: Describe the sensation.

COX: Well, it's just - it's relatively quiet, actually, so it's not too bad but ringing in the ear, for people the sounds are very different. You know, some people describe whooshing sounds, sometimes hissing, sometimes buzzing sounds. Because we don't really fully understand how they're created but it's very individual. And mine is just a sort of general high pitched tone which you can hear if I go into a very quiet space. It's not always there but it's sometimes there and it's very annoying.

GROSS: And what do doctors tell you about why you're hearing that sound?

COX: Well, yeah, and tinnitus is kind of - well, there are some debates about what actually creates tinnitus. And it could be the fact that we have connections back from our brain to our ear and it could be something to do with our brain generating that sound. So there's debates about whether the generation is in the inner ear or it's along the auditory nerve or it's somewhere in the brain.

So that's still being debated and until we know exactly what's causing it, there's not really going to be a cure for it. So it's - unfortunately, a lot of people sort of know things about hearing loss but, actually, one of the things you get when you get hearing loss is often tinnitus and that can be very distressing. It can keep people awake. It can make it difficult for them to work. So it can be a very difficult condition to live with.

Mine's mild. It's just annoying; it's not doing me any great harm.

GROSS: Does the tinnitus get amplified when you're in the anechoic chamber, the totally soundproof room?

COX: Yeah. Well, it certainly becomes noticeable. So I can't hear it now at the moment. There's just enough noise around here, even though I'm in a studio, that I can't hear it. So I have to be in very quiet conditions. It is very mild tinnitus. So, yeah, an anechoic chamber if I go - I mean, I have been to silent places. Like the desert was silent for times.

And, actually, when I was in the desert I hadn't got the tinnitus yet. I'll have to go out to the desert. So I didn't hear it then but I would imagine in very quiet places I would hear it - out in the countryside as well.

GROSS: And just one more thing. You play saxophone, so I assume you really love music. You write that music activates more of the brain than any other known stimulus. I guess maybe that's why music is just so satisfying.

COX: Yes. It is a most amazing stimulus and what's curious about it in a sense you think, well, why do we make music? I suppose the obvious answer is because we like it but then why did we develop it in the first place? You know, is it just purely a pleasure thing or is there some purpose? And there's lots of debates about what, you know, why do we have this signal which can so tap into our emotions.

I mean one of the suggestions is that it's kind of a bonding thing that enables us to socialize and, you know, bring us together in a group and that's really handy because, you know, we're a social animal and we succeed at being in groups. But there are other suggestions that basically it's almost like a parasitic thing. We could make it and therefore we did and it's just sort of pleasure. It's a bit like taking a drug: we don't need to do it but we decided it would be nice.

GROSS: Well, Trevor Cox, thank you so much for talking with us.

COX: Oh, it's been great. Thanks very much.

GROSS: Trevor Cox is the author of "The Sound Book: The Science of the Sonic Wonders of the World." You can hear some of the recordings of sonic wonders that he played for us on our website freshair.npr.org. Coming up, John Powers reviews a new documentary about a controversial Holocaust survivor directed by Claude Lanzmann who made the Holocaust documentary "Shoah." This is FRESH AIR.